This invention relates generally to a thermostat and, more particularly, to an electronic thermostat for controlling multiple temperature altering devices.
Thermostats are well known which sense temperature, compare the temperature to a preselected temperature reference and then produce an actuation signal in accordance with the comparison to control the application of power to temperature altering devices.
In some applications, such as in controlling the temperature in a hen house or hog house or other animal enclosure, it is of the utmost importance to very closely control the temperature within a narrow range around a set temperature. Accordingly, it is sometimes necessary to control multiple temperature altering devices, some or all of which are cooling devices or heating devices or a combination of both. For the same reasons, it is important to have accurate and fast temperature sensing and accurate calibration and reliable actuation of the temperature altering devices.
Practical considerations also require that the thermostat not cause the temperature altering devices to be short cycled, i.e., to be rapidly turned on and off, any more than necessary, since such short cycling causes damage to electric motors and the like and increases the wear rate on other parts. For like reasons, it is desired to prevent reapplication of power to such devices immediately after temporary power loss when power voltages may still be below rated voltages for such devices.
Various thermostat designs have been proposed to meet some of these needs, but none of them seem to be completely satisfactory in all respects. For instance, in U.S. Pat. No. 4,313,308 of Boratgis et al., a thermostat is shown which has detectors for actuating two cooling or two heating devices at different preselected temperatures, but no means are provided for back-up or override actuation of any one of the heating or cooling devices to improve reliability. Likewise, in U.S. Pat. No. 4,356,962 of Levine, a dead zone generator is provided to establish a hysteresis in the on/off characteristic of the actuation signal, but no means are provided for creation of a dead zone between stages or cycles. In U.S. Pat. No. 4,314,665 of Levine, a dead zone between heating and cooling cycles is provided, but again no means for override actuation is provided. Other electronic thermostats similar to these are also shown in U.S. Pat. Nos. 3,815,815 of Sapir and 4,240,077 of Hughes et al.